Students are often confused about when to use the prepositions “by” and “until”. Both are used to indicate the time or date something must be completed, but not later than the expressed time or date.
Here are two examples:
1. I need your edited blog post by 5 PM today.
2. You have until 5 PM today to give me your edited blog post.
Both “by” and “until” indicate the assignment can be done before but not later than 5 PM.
Citizens of the USA must file income tax returns by April 15.
Citizens of the USA have until April 15 to file personal income tax returns.
The choice of “by” or “until” depends on the type of verb. If the verb has a continuous meaning, choose “until”.
We drove until sunset and found a hotel for the night.
I worked on the presentation until 6pm and left the office to catch my train.
“Until” can also be used as a preposition or as a subordinating conjunction. A subordinating conjunction is used to link a dependent (or subordinate) clause to the main clause of a sentence. .
He worked for GM until June 2010 (until is used as a preposition).
Her child didn’t speak until she was 3 years old (until is used as a subordinating conjunction).
We use “by” when the verb in the sentence refers to a single action performed at a specific point in time. We also use by in affirmative sentences and questions.
I will call you by 8pm tonight (meaning – I will call at or before 8pm).
You must sign the contract by April 30 or we risk losing the deal.
If we finish the weekly sales meeting by 11:30, we can work on the sales presentation for the rest of the afternoon.
Will you be back in the office by 6pm?
Yes, I will return by then.
However, when a negative verb (e.g., don’t, doesn’t, won’t) is used in a sentence, we use “until”.
Helen won’t be able to finish the project plan updates until tomorrow.
Robert didn’t complete his section of the presentation until Friday.
In these examples, the verbs have a continuous meaning and the expected result was not completed until the indicated time.